Photovoltaic power generation system, photovoltaic inverter, and direct current combiner box

ABSTRACT

A photovoltaic power generation system includes a protection switch and a plurality of DC-DC converters. Each DC-DC converter includes a direct current bus, a DC-DC circuit, and at least one input interface. The input interface is configured to connect to a photovoltaic unit. The photovoltaic unit includes at least one photovoltaic module. The input interface is connected to the direct current bus by using the protection switch, the direct current bus is connected to an input end of the DC-DC circuit, and an output end of the DC-DC circuit is an output end of the DC-DC converter. The protection switch includes a release device and a switching device that are connected in series. The release device is configured to: when a short-circuit fault occurs on a line in which the release device is located, control the switching device to be disconnected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2020/142425, filed on Dec. 31, 2020. The disclosures of theaforementioned applications are hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The embodiments relate to the field of photovoltaic power generationtechnologies, a photovoltaic power generation system, a photovoltaicinverter, and a direct current combiner box.

BACKGROUND

Photovoltaic power generation is a technology that uses a photovoltaiceffect of a semiconductor interface to convert light energy intoelectric energy. A photovoltaic power generation system may usuallyinclude a photovoltaic unit, an inverter, an alternating current powerdistribution device, and the like. To obtain a relatively high outputvoltage or output current, the photovoltaic unit usually formed byconnecting a plurality of photovoltaic modules in a serial and/orparallel connection manner. To improve power generation efficiency ofthe photovoltaic power generation system, the photovoltaic unit isconnected to a component that has an independent maximum power pointtracking (MPPT) function.

Currently, to improve a direct current ratio of the photovoltaic powergeneration system (a ratio of power of the photovoltaic unit to inputpower of the photovoltaic inverter), each MPPT component is usuallyconnected to at least two photovoltaic units. In an example in which aphotovoltaic unit or a line in which a photovoltaic unit is located isshort circuited, a short-circuit current is a sum of output currents ofother connected photovoltaic units. When there is only one anotherconnected photovoltaic unit, the photovoltaic unit and the line cantolerate this short-circuit current because the short-circuit current isrelatively small. However, when there are two or more another connectedphotovoltaic units, the short-circuit current is relatively large. Toprotect the photovoltaic unit and the line, a fuse may be connected inseries to a positive output end or a negative output end of thephotovoltaic unit, so that the fuse blows to protect the photovoltaicunit and the line.

However, a fusing current of the fuse is usually relatively high, and anoutput current of each photovoltaic unit is relatively low. Therefore, asum of short-circuit currents of the plurality of photovoltaic units maybe difficult to reach the fusing current of the fuse. As a result, atime consumed for blowing of the fuse is long, and the fuse cannoteffectively protect the photovoltaic unit and the line.

SUMMARY

The embodiments may provide a photovoltaic power generation system, aphotovoltaic inverter, and a direct current combiner box, to effectivelyprotect a photovoltaic unit and a line when a short circuit occurs onthe photovoltaic unit or the line.

According to a first aspect, the embodiments may provide a photovoltaicpower generation system. The photovoltaic power generation systemincludes a protection switch and a plurality of direct current(DC)-direct current converters. Each DC-DC converter includes a directcurrent bus, a DC-DC circuit, and at least one input interface. Theinput interface is configured to connect to a photovoltaic unit, and thephotovoltaic unit includes at least one photovoltaic module. The inputinterface is connected to the direct current bus by using the protectionswitch, the direct current bus is connected to an input end of the DC-DCcircuit, and an output end of the DC-DC circuit is an output end of theDC-DC converter. The protection switch includes a release device and aswitching device that are connected in series. The release device isconfigured to: when a short-circuit fault occurs on a line in which therelease device is located, control the switching device to bedisconnected.

According to the photovoltaic power generation system, when ashort-circuit fault occurs on the photovoltaic unit, the release devicecontrols the switching device to be disconnected, so that the interfaceis disconnected from the direct current bus, and the photovoltaic unitconnected to the interface is disconnected from the direct current bus.Therefore, a photovoltaic unit connected to another interface does notoutput a current to a line in which the photovoltaic unit with theshort-circuit fault is located, thereby protecting the photovoltaic unitand the line from damage. Based on a protection action triggered by theswitching device under control by the release device, no additionalcontrol circuit is required, and implementation difficulty of thesolution is reduced. In addition, because a fuse is no longer used, a Ywire harness originally used for the built-in fuse may be disposed on aphotovoltaic unit side instead of being disposed below a photovoltaicinverter or a direct current combiner box of the photovoltaic powergeneration system, so that cable costs are further reduced.

In a possible implementation, the release device is an electromagneticrelease device. When a reverse current on the line in which the releasedevice is located is greater than a first current value, the releasedevice controls the switching device to be disconnected. The firstcurrent value is related to an electrical parameter of theelectromagnetic release device.

In a possible implementation, the release device is an electromagneticrelease device. When an overcurrent occurs on the line in which therelease device is located, the release device controls the switchingdevice to be disconnected.

In a possible implementation, the release device is a thermal releasedevice. When an overcurrent occurs on the line in which the releasedevice is located, the release device controls the switching device tobe disconnected. For example, for a bimetallic sheet thermal releasedevice, when an overcurrent occurs on a line in which the release deviceis located, a bimetallic sheet generates heat to drive the switchingdevice to act.

In a possible implementation, each input interface is connected to onephotovoltaic unit.

In a possible implementation, a maximum of two photovoltaic units areconnected in parallel, and then connected to the input interface. Inthis case, each photovoltaic unit can tolerate a current input by one ortwo other photovoltaic units.

In a possible implementation, a maximum of three photovoltaic units areconnected in parallel, and then connected to the input interface. Inthis case, each photovoltaic unit can tolerate currents input by amaximum of two other photovoltaic units.

In a possible implementation, the photovoltaic power generation systemfurther includes a direct current-alternating current (AC) circuit, anda DC-AC converter and the plurality of DC-DC converters form aninverter. Positive output ports of the plurality of DC-DC converters areconnected in parallel to a positive input port of the DC-AC converter,negative output ports of the plurality of DC-DC converters are connectedin parallel to a negative input port of the DC-AC converter, and anoutput port of the DC-AC converter is an output port of the inverter.

In a possible implementation, the plurality of DC-DC converters form adirect current combiner box, the positive output ports of the pluralityof DC-DC converters are connected in parallel to form a positive outputport of the direct current combiner box, and the negative output portsof the plurality of DC-DC converters are connected in parallel to form anegative output port of the direct current combiner box.

In a possible implementation, the photovoltaic power generation systemfurther includes a protective device. The protective device is connectedin series or in parallel to the photovoltaic unit. The release device isfurther configured to: when controlling the switching device to bedisconnected, prevent the protective device from triggering a protectionaction.

In other words, when the current photovoltaic power generation systemthat uses the protective device is reconstructed, the protective devicemay not need to be removed, so as to facilitate reconstruction.

In a possible implementation, the protective device includes at leastone of a fuse, an optimizer, or a disconnection box.

According to a second aspect, the embodiments may further provide aphotovoltaic inverter, configured to connect to a photovoltaic unit. Thephotovoltaic unit includes at least one photovoltaic module, and thephotovoltaic inverter includes a protection switch, a DC-AC converter,and a plurality of DC-DC converters. Each DC-DC converter includes adirect current bus, a DC-DC circuit, and at least one input interface.The input interface is configured to connect to the photovoltaic unit,and the photovoltaic unit includes at least one photovoltaic module. Theinput interface is connected to the direct current bus by using theprotection switch, the direct current bus is connected to an input endof the DC-DC circuit, and an output end of the DC-DC circuit is anoutput end of the DC-DC converter. Positive ports of the output ends ofthe plurality of DC-DC converters are connected in parallel to apositive input port of the DC-AC converter, and negative ports of theoutput ends of the plurality of DC-DC converters are connected inparallel to a negative input port of the DC-AC converter. The protectionswitch includes a release device and a switching device that areconnected in series. The release device is configured to: when ashort-circuit fault occurs on a line in which the release device islocated, control the switching device to be disconnected.

According to the photovoltaic inverter, when a short-circuit faultoccurs on the connected photovoltaic unit, the release device controlsthe switching device to be disconnected, so that the interface isdisconnected from the direct current bus, and the photovoltaic unitconnected to the interface is disconnected from the direct current bus.Therefore, a photovoltaic unit connected to another interface does notoutput a current to a line in which the photovoltaic unit with theshort-circuit fault is located, thereby protecting the photovoltaic unitand the line from damage. Based on a protection action triggered by theswitching device under control by the release device, no additionalcontrol circuit is required, and implementation difficulty of thesolution is reduced. In addition, because a fuse is no longer used, a Ywire harness originally used for a built-in fuse may be disposed on aphotovoltaic unit side instead of being disposed below the photovoltaicinverter of the photovoltaic power generation system. Therefore, cablecosts are further reduced.

With reference to the second aspect, in a possible implementation, therelease device is an electromagnetic release device. When a reversecurrent on the line in which the release device is located is greaterthan a first current value, the release device controls the switchingdevice to be disconnected.

With reference to the second aspect, in a possible implementation, therelease device is an electromagnetic release device. When an overcurrentoccurs on the line in which the release device is located, the releasedevice controls the switching device to be disconnected.

With reference to the second aspect, in a possible implementation, therelease device is a thermal release device. When an overcurrent occurson the line in which the release device is located, the release devicecontrols the switching device to be disconnected.

According to a third aspect, the embodiments may further provide adirect current combiner box, configured to connect to a photovoltaicunit. The photovoltaic unit includes at least one photovoltaic module,and the direct current combiner box includes a protection switch and aplurality of DC-DC converters. Each DC-DC converter includes a directcurrent bus, a DC-DC circuit, and at least one input interface. Theinput interface is configured to connect to the photovoltaic unit, andthe photovoltaic unit includes at least one photovoltaic module. Theinput interface is connected to the direct current bus by using theprotection switch, the direct current bus is connected to an input endof the DC-DC circuit, and an output end of the DC-DC circuit is anoutput end of the DC-DC converter. Positive ports of the output ends ofthe plurality of DC-DC converters are connected in parallel to apositive output port of the direct current combiner box, and negativeports of the output ends of the plurality of DC-DC converters areconnected in parallel to a negative output port of the direct currentcombiner box. The protection switch includes a release device and aswitching device that are connected in series. The release device isconfigured to: when a short-circuit fault occurs on a line in which therelease device is located, control the switching device to bedisconnected.

According to the direct current combiner box, when a short-circuit faultoccurs on the connected photovoltaic unit, the release device controlsthe switching device to be disconnected, so that the interface isdisconnected from the direct current bus, and the photovoltaic unitconnected to the interface is disconnected from the direct current bus.Therefore, a photovoltaic unit connected to another interface does notoutput a current to a line in which the photovoltaic unit with theshort-circuit fault is located, thereby protecting the photovoltaic unitand the line from damage. Based on a protection action triggered by theswitching device under control by the release device, no additionalcontrol circuit is required, and implementation difficulty of thesolution is reduced. In addition, because a fuse is no longer used, a Ywire harness originally used for a built-in fuse may be disposed on aphotovoltaic unit side instead of being disposed below the directcurrent combiner box of the photovoltaic power generation system.Therefore, cable costs are further reduced.

With reference to the third aspect, in a possible implementation, therelease device is an electromagnetic release device. When a reversecurrent on the line in which the release device is located is greaterthan a first current value, the release device controls the switchingdevice to be disconnected.

With reference to the third aspect, in a possible implementation, therelease device is an electromagnetic release device. When an overcurrentoccurs on the line in which the release device is located, the releasedevice controls the switching device to be disconnected.

With reference to the third aspect, in a possible implementation, therelease device is a thermal release device. When an overcurrent occurson the line in which the release device is located, the release devicecontrols the switching device to be disconnected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram 1 of a short-circuit protection circuitused in a conventional technology;

FIG. 2 is a schematic diagram 2 of a short-circuit protection circuitused in a conventional technology;

FIG. 3 is a schematic diagram 3 of a short-circuit protection circuitused in a conventional technology

FIG. 4 is a schematic diagram of a branch according to an embodiment;

FIG. 5 is a schematic diagram of another branch according to anembodiment;

FIG. 6 is a schematic diagram of a photovoltaic power generation systemaccording to an embodiment;

FIG. 7 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment;

FIG. 8 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment;

FIG. 9 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment;

FIG. 10 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment;

FIG. 11 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment;

FIG. 12 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment;

FIG. 13 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment;

FIG. 14 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment;

FIG. 15 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment;

FIG. 16 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment;

FIG. 17 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment;

FIG. 18 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment;

FIG. 19 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment;

FIG. 20 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment;

FIG. 21 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment;

FIG. 22 is a schematic diagram of a photovoltaic inverter according toan embodiment; and

FIG. 23 is a schematic diagram of a direct current combiner boxaccording to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To improve a direct current ratio of a photovoltaic power generationsystem, each MPPT component is usually connected to at least twophotovoltaic units or more photovoltaic units. In addition, to protect aphotovoltaic unit and a line when the photovoltaic unit or the line isshort circuited, a positive output end or a negative output end of thephotovoltaic unit is connected in series to a fuse (or referred to as afuse blowing). The following describes an example in which each MPPTcomponent is connected to three branches. A principle when each MPPTcomponent is connected to more branches is similar. Details are notdescribed herein again.

Refer to all of FIG. 1 to FIG. 3. FIG. 1 is a schematic diagram in whichboth a positive output end and a negative output end of a photovoltaicunit are connected in series to fuses. FIG. 2 is a schematic diagram inwhich a positive output end of a photovoltaic unit is connected inseries to a fuse. FIG. 3 is a schematic diagram in which a negativeoutput end of a photovoltaic unit is connected in series to a fuse.

Each branch includes one photovoltaic module 101. Three branches areconnected in parallel before a switch 102 and then are connected to anMPPT component 103 by using the direct current switch 102. A fuse 1 to afuse 6 in FIG. 1, a fuse 1 to a fuse 3 in FIG. 2, and a fuse 1 to a fuse3 in FIG. 3 are fuses that blow when a current in a line is excessivelylarge to protect the photovoltaic module and the line.

However, because an actual output current of the photovoltaic unit isrelatively small, the fuse cannot blow fast. A fuse whose rated currentis 15 A is used as an example. According to a fuse standard, when thefuse does not blow, an allowed current may reach up to 1.13×15=16.95 A,and a current required for the fuse to blow within one hour is1.35×15=20.25 A. A short-circuit current is difficult to meet thecurrent required for fast blowing of the fuse. Therefore, the fuse maynot blow or needs a relatively long time to blow. As a result, aphotovoltaic unit and a line cannot be effectively protected. In someembodiments, because cable protection needs to be considered, a Y wireharness of a built-in fuse further needs to be disposed on a DC-DCconverter side, and a photovoltaic unit of a DC-DC converter may beconnected by using a relatively long cable. This further increases cablecosts.

To resolve the foregoing problem, the embodiments may provide aphotovoltaic power generation system, a photovoltaic inverter, and adirect current combiner box, to effectively protect a photovoltaic unitand a line when a short circuit occurs on the photovoltaic unit or theline. Details are described below with reference to the accompanyingdrawings.

The following terms “first”, “second”, and the like are merely intendedfor a purpose of description and shall not be understood as anindication or implication of relative importance or implicit indicationof a quantity of indicated features. Therefore, a feature limited by“first”, “second”, or the like may explicitly or implicitly include oneor more features.

To make a person skilled in the art understand the solutions better, thefollowing clearly describes the embodiments with reference to theaccompanying drawings.

A single photovoltaic unit in the following embodiments may include onephotovoltaic module or may be formed by connecting a plurality ofphotovoltaic modules in series and/or in parallel. For example, aplurality of photovoltaic modules is first connected in series to form aphotovoltaic string, and then a plurality of photovoltaic strings isconnected in parallel to form a photovoltaic unit. A quantity ofphotovoltaic modules included in the photovoltaic unit is not limited inthis embodiment and may be set by a person skilled in the art based onan actual requirement. In addition, an electrical parameter of a singlephotovoltaic module is not limited in this embodiment.

Output voltages of a plurality of photovoltaic units connected to a sameDC-DC converter may be the same or different. This is not limited inthis embodiment.

The DC-DC converter of the photovoltaic power generation system providedin this embodiment can be connected to at least two photovoltaic unitsby using an interface. After being connected by using the interface, thephotovoltaic units may be connected in parallel inside the DC-DCconverter to a direct current bus, so that output currents of thephotovoltaic units are aggregated into the direct current bus, therebyforming a branch. The following first describes an existing form of thebranch.

FIG. 4 is a schematic diagram of a branch according to an embodiment.

The branch includes a photovoltaic unit 101 a 1, a positive output endof the photovoltaic unit 101 a 1 is a positive output end of the branch,and a negative output end of the photovoltaic unit 101 a 1 is a negativeoutput end of the branch. This is not described in description of thefollowing embodiments.

FIG. 5 is a schematic diagram of another branch according to anembodiment.

The branch may include a plurality of branches shown in FIG. 4.Therefore, at least two photovoltaic units are included, for example,101 a 1, 101 a 2, . . . , and 101 ai in sequence.

The branch in this embodiment is a concept in the electricity field andrefers to a line through which a branch current to be aggregated into adirect current bus flows. Still using FIG. 5 as an example, a line inwhich the photovoltaic unit 101 a 1 is located may be referred to as abranch, and a line formed by connecting the photovoltaic unit 101 a 1and the photovoltaic unit 101 a 2 in parallel may also be referred to asa branch. Positive output ends of all photovoltaic units are aggregatedto form a positive output end of the branch, and negative output ends ofall the photovoltaic units are aggregated to form a negative output endof the branch.

The “branch” in the following embodiments refers to a general term ofall branches shown in FIG. 4 and FIG. 5, in other words, a general termof all branches except a trunk (the direct current bus).

The following provides description by using an example in which aphotovoltaic power generation system includes one DC-DC converter. Whenthe photovoltaic power generation system includes a plurality of DC-DCconverters, a principle is similar, and details are not described.

FIG. 6 is a schematic diagram of a photovoltaic power generation systemaccording to an embodiment.

The photovoltaic power generation system includes photovoltaic units101, protection switches S₁ to S_(M+N), and a DC-DC converter 200.

The DC-DC converter 200 includes an interface, a direct current bus, anda DC-DC circuit 201.

The DC-DC converter 200 may be connected to the photovoltaic units byusing the interface. A quantity of photovoltaic units that are connectedto a same interface is not limited.

When one interface is connected to a plurality of photovoltaic units,the plurality of photovoltaic units is connected in parallel to form thebranch shown in FIG. 5 and then connected to the interface.

Each photovoltaic unit includes at least one photovoltaic module.

Each protection switch includes a release device and a switching devicethat are connected in series. The release device is configured to: whena short-circuit fault occurs on a line in which the release device islocated, control the switching device to be disconnected. In otherwords, in this case, the protection switch is disconnected, and an inputinterface of a line in which the protection switch is located isdisconnected from the direct current bus, to cut off the line with theshort-circuit fault.

Using FIG. 6 as an example, when a short-circuit fault occurs on asingle photovoltaic unit and the faulty photovoltaic unit can toleratean output current of one another normal photovoltaic unit, a value of iin the figure is 2.

For another example, when a short-circuit fault occurs on a singlephotovoltaic unit and the faulty photovoltaic unit can tolerate outputcurrents of two other normal photovoltaic units, the value of i in thefigure is 3.

A value of i is determined based on an actual current tolerance value ofthe photovoltaic unit. This is not limited in this embodiment. It shouldbe noted that, the illustration in FIG. 6 is merely for ease of drawingand description. The i photovoltaic units in the figure may be connectedin parallel inside the DC-DC converter 200 or may be first externallyconnected in parallel to an interface for connecting the direct currentbus.

When there is no short-circuit fault, currents of all branches areaggregated into the direct current bus. Therefore, an absolute value ofa current of the direct current bus is greater than an absolute value ofa current of any branch. A current direction is flowing from a positiveelectrode of a photovoltaic unit to a positive direct current bus.

When a short-circuit fault occurs on any branch, output currents of allother normal branches flow to the branch with the short-circuit fault,and a reverse current occurs on the branch with the short-circuit fault.When a value of M+N is greater than 2, an overcurrent further occurs onthe branch with the short-circuit fault.

The release device is mechanically connected to the switching device andis configured to release a holding mechanism when a protection action istriggered, so that the switching device is automatically disconnected. Aprinciple of the release device is as follows: When a reverse current oran overcurrent occurs on a branch in which the release device islocated, the release device controls the switching device to bedisconnected, so as to protect the photovoltaic unit and the line.

The DC-DC circuit 201 may be a boost circuit, a buck circuit, or abuck-boost circuit.

In conclusion, according to the photovoltaic power generation system,when a short-circuit fault occurs on a photovoltaic unit, the releasedevice controls the switching device to be disconnected, so that theinterface is disconnected from the direct current bus, and thephotovoltaic unit connected to the interface is disconnected from thedirect current bus. Therefore, a photovoltaic unit connected to anotherinterface does not output a current to a line in which the photovoltaicunit with the short-circuit fault is located, thereby protecting thephotovoltaic unit and the line from damage. Based on a protection actiontriggered by the switching device under control by the release device,no additional control circuit is required, and implementation difficultyof the solution is reduced. In addition, because a fuse is no longerused, a Y wire harness originally used for the built-in fuse may bedisposed on a photovoltaic unit side instead of being disposed below aphotovoltaic inverter or a direct current combiner box of thephotovoltaic power generation system, so that cable costs are furtherreduced.

The release device may be implemented in different manners. For example:In some embodiments, the release device is an electromagnetic releasedevice. When a reverse current on the branch in which the release deviceis located is greater than a first current value, the release devicecontrols the switching device to be disconnected. In some otherembodiments, the release device is an electromagnetic release device.When an overcurrent occurs on the branch in which the release device islocated, the release device controls the switching device to bedisconnected. In still some other embodiments, the release device is athermal release device. When an overcurrent occurs on the branch inwhich the release device is located, the release device controls theswitching device to be disconnected.

The following describes an implementation of the photovoltaic powergeneration system.

The following first provides description by using an example in which aDC-DC converter is connected to two photovoltaic units.

FIG. 7 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment.

Each DC-DC converter 200 is connected to two photovoltaic units 101 a 1and 101 a 2 by using an interface.

The two photovoltaic units are connected to the interface of the DC-DCconverter 200. After being connected in parallel inside the DC-DCconverter 200, the two photovoltaic units are connected to a DC-DCcircuit 201 by using a direct current switch 102. The direct currentswitch 102 is configured to protect the circuit. In some embodiments, adirect connection may be performed instead of disposing the directcurrent switch 102.

Each photovoltaic unit is further connected in series to a protectionswitch S1.

When there is no short-circuit fault, currents of the two photovoltaicunits are aggregated into a direct current bus. An absolute value of acurrent of the direct current bus (an absolute value of a detectioncurrent of a point A or a point B) is greater than an absolute value ofa current of any branch (an absolute value of a detection current of apoint C or a point D).

When a short-circuit fault occurs on one photovoltaic unit, an outputcurrent of the other normal photovoltaic unit flows to theshort-circuited photovoltaic unit. As a result, a reverse current occurson a branch in which the photovoltaic unit with the short-circuit faultis located.

In this case, a release device may be an electromagnetic release device.When a reverse current on a branch in which the release device islocated is greater than a first current value, the release devicecontrols a switching device to be disconnected.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 1 is located, a protection switch S1 is disconnected todisconnect the branch in which the photovoltaic unit 101 a 1 is located,thereby protecting the photovoltaic unit and the line. When ashort-circuit fault occurs on a branch in which the photovoltaic unit101 a 2 is located, a protection switch S2 is disconnected to disconnectthe branch in which the photovoltaic unit 101 a 2 is located, therebyprotecting the photovoltaic unit and the line.

In some embodiments, a protection switch may be connected in series to apositive output end of a photovoltaic unit or may be connected in seriesto a negative output end of the photovoltaic unit. Alternatively, oneprotection switch may be connected in series to each of a positiveoutput end and a negative output end of a photovoltaic unit to implementredundancy control. This is not limited in this embodiment.

In some other embodiments, when one DC-DC circuit is connected to onlytwo photovoltaic units, either of the protection switches S1 and S2 maybe canceled. For example, after S1 is canceled, when the photovoltaicunit 101 a 1 has a short circuit, the photovoltaic unit 101 a 2transmits a current to the branch in which the photovoltaic unit 101 a 1is located, but the current is within a tolerance range of thephotovoltaic unit 101 a 1. Therefore, the photovoltaic unit 101 a 1 isnot damaged. When the photovoltaic unit 101 a 2 has a short circuit, theprotection switch S2 is disconnected to protect the circuit.

In conclusion, when the switching device of the photovoltaic powergeneration system is disconnected, the interface is disconnected fromthe direct current bus, and the photovoltaic unit connected to theinterface is disconnected from the direct current bus. Therefore, aphotovoltaic unit connected to another interface does not output acurrent to a line in which the photovoltaic unit with the short-circuitfault is located, thereby protecting the photovoltaic unit and the linefrom damage. Based on a protection action triggered by the switchingdevice under control by the release device, no additional controlcircuit is required, and implementation difficulty of the solution isreduced. In addition, because a fuse is no longer used, a Y wire harnessoriginally used for the built-in fuse may be disposed on a photovoltaicunit side instead of being disposed below a photovoltaic inverter or adirect current combiner box of the photovoltaic power generation system,so that cable costs are further reduced.

The foregoing embodiment is described by using an example in which eachDC-DC converter is connected to photovoltaic units by using two inputinterfaces. However, currently, to improve a direct current ratio of thephotovoltaic power generation system, three, four, and even more inputinterfaces may be usually disposed in each DC-DC converter to connectphotovoltaic units. The following first describes a working principlefor a case that three input interfaces are disposed in each DC-DCconverter.

FIG. 8 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment.

Three photovoltaic units are respectively connected to the three inputinterfaces of the DC-DC converter. After the three photovoltaic unitseach is connected in series to one protection switch inside the DC-DCconverter, the three photovoltaic units are connected in parallel andthen connected to a DC-DC circuit 201 by using a direct current switch102. The direct current switch 102 is configured to protect the circuit.In actual application, a direct connection may be performed instead ofdisposing the direct current switch 102.

A photovoltaic unit 101 a 1 is connected in series to a protectionswitch S1, a photovoltaic unit 101 a 2 is connected in series to aprotection switch S2, and a photovoltaic unit 101 a 3 is connected inseries to a protection switch S3.

When there is no short-circuit fault, output currents of the threephotovoltaic units are aggregated into a direct current bus. Therefore,an absolute value of a current of the direct current bus (an absolutevalue of a detection current of a point A or a point B) is greater thanan absolute value of a current of any branch (an absolute value of adetection current of a point C, a point D, or a point E). In this case,a current direction of the point C, the point D, and the point E may beset as a preset current direction, for example, set as a positivedirection.

When a short-circuit fault occurs on one photovoltaic unit, it isassumed that the short-circuit fault occurs on a line in which thephotovoltaic unit 101 a 3 is located and a single photovoltaic unit cantolerate a reverse current transmitted from only one another branch.

In this case, output currents of the other two normal photovoltaic unitsflow to the photovoltaic unit 101 a 3 with the short circuit fault. As aresult, an overcurrent occurs on a branch in which the photovoltaic unit101 a 3 is located and a reverse current occurs (opposite to the currentdirection of the point E).

In a possible implementation, a release device of S3 is anelectromagnetic release device. When the reverse current on the branchin which the photovoltaic unit 101 a 3 is located is greater than afirst current value, the release device controls a switching device tobe disconnected.

In another possible implementation, a release device of S2 is anelectromagnetic release device. When an overcurrent occurs on the branchin which the photovoltaic unit 101 a 3 is located, the release devicecontrols a switching device to be disconnected.

In still another possible implementation, the release device is athermal release device. When an overcurrent occurs on the branch inwhich the photovoltaic unit 101 a 3 is located, the release devicecontrols a switching device to be disconnected.

One input interface may alternatively be connected in series to twoprotection switches and then to the direct current bus inside the DC-DCconverter. In this case, the two protection switches may use differenttypes of release devices.

After S3 is disconnected, the photovoltaic unit 101 a 1 and thephotovoltaic unit 101 a 2 can continue to work normally, therebyprotecting the photovoltaic unit and the line.

The foregoing embodiment is described by using an example in which threeinput interfaces are disposed in each DC-DC converter. The followingdescribes a principle for a case that four input interfaces arecorrespondingly disposed in each DC-DC converter.

FIG. 9 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment.

Each of four photovoltaic units is connected to one input port of theDC-DC converter and then connected in series to a protection switch.Subsequently, the four photovoltaic units are connected in parallel to aDC-DC circuit 201.

Positive output ends of the photovoltaic units are aggregated and thenconnected to a protection switch S1. A negative output end of aphotovoltaic unit 101 a 1 is connected to a protection switch S3. Anegative output end of a photovoltaic unit 101 a 2 is connected to aprotection switch S2. A negative output end of a photovoltaic unit 101 a3 is connected to a protection switch S4. A negative output end of aphotovoltaic unit 101 a 4 is connected to a protection switch S5.

In some embodiments, a direct connection may be performed instead ofdisposing the protection switch S1 in FIG. 9.

When there is no short-circuit fault, currents of the four photovoltaicunits are aggregated into the direct current bus, and therefore anabsolute value of a current of the direct current bus (an absolute valueof a detection current of a detection point A or B) is greater than anabsolute value of a current of any branch (absolute values of detectioncurrents of detection points C, D, E, and F).

When a short-circuit fault occurs on one photovoltaic unit, outputcurrents of the other normal photovoltaic units flow to the photovoltaicunit with the short-circuit fault. As a result, an overcurrent and areverse current occur on a faulty branch.

That a single photovoltaic unit can tolerate a current flowing from onlyone another branch is still used as an example. In this case, a releasedevice of a protection switch on the faulty branch controls acorresponding switching device to be disconnected, so that the faultybranch is disconnected, and the other photovoltaic units can continue towork normally, thereby protecting the photovoltaic unit and the line.

FIG. 10 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment.

A difference between an implementation shown in FIG. 10 and that in FIG.9 lies in: A positive output end of a photovoltaic unit 101 a 1 and apositive output end of a photovoltaic unit 101 a 2 are aggregated at aprotection switch S1 and are connected to a positive direct current busby using the protection switch S1. A positive output end of aphotovoltaic unit 101 a 3 and a positive output end of a photovoltaicunit 101 a 4 are aggregated at a protection switch S6 and are connectedto the positive direct current bus by using the protection switch S6.

In this case, a principle is similar to that in FIG. 9, and details arenot described in this embodiment.

FIG. 11 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment.

A difference between a manner shown in FIG. 11 and that in FIG. 9 liesin: A positive output end of each photovoltaic unit is connected inseries to one protection switch inside a DC-DC converter and thenaggregated at a positive direct current bus. A negative output end ofeach photovoltaic unit is connected in series to one protection switchinside the DC-DC converter and then aggregated at a negative directcurrent bus. Redundantly disposing the protection switch can furtherimprove safety and ensure that a branch in which a photovoltaic unit islocated can be disconnected. A principle is similar to that in FIG. 9,and details are not described in this embodiment.

In conclusion, when the DC-DC converter of the photovoltaic powergeneration system is connected to four photovoltaic units by usinginterfaces, and a short-circuit fault occurs on a branch, a releasedevice controls a switching device to be disconnected, so that aninterface is disconnected from a direct current bus, and a photovoltaicunit connected to the interface is disconnected from the direct currentbus. Therefore, a photovoltaic unit connected to another interface doesnot output a current to a line in which the photovoltaic unit with theshort-circuit fault is located, thereby protecting the photovoltaic unitand the line from damage. Based on a protection action triggered by theswitching device under control by the release device, no additionalcontrol circuit is required, and implementation difficulty of thesolution is reduced. In addition, because a fuse is no longer used, a Ywire harness originally used for the built-in fuse may be disposed on aphotovoltaic unit side instead of being disposed below a photovoltaicinverter or a direct current combiner box of the photovoltaic powergeneration system, so that cable costs are further reduced.

FIG. 12 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment.

Photovoltaic units 101 a 1 and 101 a 2 are connected in parallel insidea DC-DC converter and then are connected to a direct current bus of theDC-DC converter by using protection switches. Branches in whichphotovoltaic units 101 a 3 and 101 a 4 are respectively located each areconnected in series to one protection switch and then are connected tothe direct current bus of the DC-DC converter.

Positive output ends of the photovoltaic units 101 a 1 and 101 a 2 areconnected to a positive direct current bus by using a protection switchS1, and negative output ends of the photovoltaic units 101 a 1 and 101 a2 are connected to a negative direct current bus by using a protectionswitch S2.

In some embodiments, a direct connection may be performed instead ofdisposing the protection switch S2.

When there is no short-circuit fault, currents of all branches areaggregated into the direct current bus, and therefore an absolute valueof a current of the direct current bus (an absolute value of a detectioncurrent of a detection point A or B) is greater than an absolute valueof a current of any branch (absolute values of detection currents ofdetection points C, D, E, F, G, and H).

When a short-circuit fault occurs on a branch, an output current of anormal branch flows to the branch with the short-circuit fault. As aresult, a reverse current occurs on the branch with the short-circuitfault.

A release device may be an electromagnetic release device. In this case,a release device of a protection switch on the faulty branch controls acorresponding switching device to be disconnected, so that the branchwith the short-circuit fault is disconnected, and another photovoltaicunit can continue to work normally, thereby protecting the photovoltaicunit and the line.

Description with reference to FIG. 12 is as follows:

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 3 is located, S3 is disconnected. In this case, thephotovoltaic unit 101 a 3 is cut off, and the photovoltaic units 101 a1, 101 a 2, and 101 a 4 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 4 is located, S4 is disconnected. In this case, thephotovoltaic unit 101 a 4 is cut off, and the photovoltaic units 101 a1, 101 a 2, and 101 a 3 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 1 is located, S1 and S2 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 2 is within a tolerancerange of the photovoltaic unit 101 a 1, and the photovoltaic units 101 a3 and 101 a 4 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 2 is located, S1 and S2 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 1 is within a tolerancerange of the photovoltaic unit 101 a 2, and the photovoltaic units 101 a3 and 101 a 4 can work normally.

FIG. 13 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment.

A difference between an implementation shown in FIG. 13 and that in FIG.12 lies in: Positive output ends of the photovoltaic units 101 a 3 and101 a 4 are aggregated and then are connected to the positive directcurrent bus by using a protection switch S3, a negative output end ofthe photovoltaic unit 101 a 3 is connected to the negative directcurrent bus by using a protection switch S4, and a negative output endof the photovoltaic unit 101 a 4 is connected to the negative directcurrent bus by using a protection switch S5.

A working principle is similar to the description corresponding to FIG.12, and details are not described in this embodiment.

FIG. 14 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment.

A difference between an implementation shown in FIG. 14 and that in FIG.13 lies in: Photovoltaic units 101 a 1 and 101 a 2 are connected inparallel, positive output ends of the photovoltaic units 101 a 1 and 101a 2 are aggregated and then are connected to a positive direct currentbus by using a protection switch S1, and negative output ends thereofare aggregated and then are connected to a negative direct current busby using a protection switch S4. A positive output end of a photovoltaicunit 101 a 3 is connected to the positive direct current bus by usingthe protection switch S1, and a negative output end thereof is connectedto the negative direct current bus by using a protection switch S2. Apositive output end of a photovoltaic unit 101 a 4 is connected to thepositive direct current bus by using a protection switch S3, and anegative output end thereof is connected to the negative direct currentbus by using the protection switch S4.

A working principle is similar to the description corresponding to FIG.12, and details are not described in this embodiment.

FIG. 15 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment.

Photovoltaic units 101 a 1 and 101 a 2 are directly connected inparallel inside a DC-DC converter. Positive output ends of thephotovoltaic units 101 a 1 and 101 a 2 are connected to a positivedirect current bus by using a protection switch S1, and negative outputends thereof are connected to a negative direct current bus by using aprotection switch S2.

Photovoltaic units 101 a 3 and 101 a 4 are directly connected inparallel inside the DC-DC converter. Positive output ends of thephotovoltaic units 101 a 3 and 101 a 4 are connected to the positivedirect current bus by using a protection switch S3, and negative outputends thereof are connected to the negative direct current bus by using aprotection switch S4.

When there is no short-circuit fault, currents of all branches areaggregated into the direct current bus, and therefore an absolute valueof a current of the direct current bus (an absolute value of a detectioncurrent of a detection point A or B) is greater than an absolute valueof a current of any branch (absolute values of detection currents ofdetection points C, D, E, F, G, and H).

When a short-circuit fault occurs on a branch, an output current of anormal branch flows to the branch with the short-circuit fault. As aresult, a reverse current occurs on the branch with the short-circuitfault.

A release device may be an electromagnetic release device. In this case,a release device of a protection switch on the faulty branch controls acorresponding switching device to be disconnected, so that the branchwith the short-circuit fault is disconnected, and another photovoltaicunit can continue to work normally, thereby protecting the photovoltaicunit and the line.

Description with reference to FIG. 15 is as follows:

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 1 is located, S1 and S2 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 2 is within a tolerancerange of the photovoltaic unit 101 a 1, and the photovoltaic units 101 a3 and 101 a 4 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 2 is located, S1 and S2 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 1 is within a tolerancerange of the photovoltaic unit 101 a 2, and the photovoltaic units 101 a3 and 101 a 4 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 3 is located, S3 and S4 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 4 is within a tolerancerange of the photovoltaic unit 101 a 3, and the photovoltaic units 101 a1 and 101 a 2 can work normally.

When a short-circuit fault occurs on a branch in which the photovoltaicunit 101 a 4 is located, S3 and S4 are disconnected. In this case, acurrent input by the photovoltaic unit 101 a 3 is within a tolerancerange of the photovoltaic unit 101 a 4, and the photovoltaic units 101 a1 and 101 a 2 can work normally.

In some embodiments, a direct connection may be performed instead ofdisposing at least one of the protection switches S1 and S2, or a directconnection may be performed instead of disposing at least one of theprotection switches S3 and S4, or a direct connection may be performedinstead of disposing either of the protection switches S1 and S2 andeither of the protection switches S3 and S4, to reduce a quantity ofprotection switches connected in series, thereby reducing costs.

The foregoing embodiments describe the working principles for the casesthat each DC-DC converter includes three input interfaces and the casesthat each DC-DC converter includes four input interfaces. In someembodiments, each DC-DC converter may further be correspondinglyconnected to more photovoltaic units. The following describes a workingprinciple for a case that a quantity of photovoltaic units connected toeach DC-DC converter is greater than four.

FIG. 16 is a schematic diagram of another photovoltaic power generationsystem according to an embodiment.

A DC-DC converter is provided with M input interfaces to connect tophotovoltaic units, and each interface is connected to one photovoltaicunit to form M first-type photovoltaic unit branches. M is an integergreater than or equal to 3.

When the photovoltaic unit is connected in series to one protectionswitch, the protection switch is connected in series to a positiveoutput end or a negative output end of the photovoltaic unit. When thephotovoltaic unit is connected in series to two protection switches, theprotection switches are connected in series to a positive output end anda negative output end of the photovoltaic unit to implement redundancyprotection.

The M photovoltaic units are each connected in series to one protectionswitch, and then are connected in parallel to a direct current bus ofthe DC-DC converter.

When there is no short-circuit fault, currents of all photovoltaic unitsare aggregated into the direct current bus, and an absolute value of acurrent of the direct current bus (an absolute value of a detectioncurrent of a detection point A or a detection point B) is greater thanan absolute value of a current of any branch.

When a short-circuit fault occurs on one photovoltaic unit, an outputcurrent of another normal photovoltaic unit flows to a branch in whichthe short-circuited photovoltaic unit is located. As a result, anovercurrent and a reverse current occur on the faulty branch.

In this case, a release device on the faulty branch controls acorresponding switching device to be disconnected, so that the faultybranch is disconnected, and another photovoltaic unit can continue towork normally.

FIG. 17 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment.

Every i photovoltaic units are directly connected in parallel inside aDC-DC converter. Subsequently, the i photovoltaic units are connected inseries to at least one protection switch and then are connected to adirect current bus of the DC-DC converter. N is an integer greater thanor equal to 2.

When a short-circuit fault occurs in a single photovoltaic unit, whenthe faulty photovoltaic unit can tolerate an output current of one othernormal photovoltaic unit, a value of i is 2; or when the faultyphotovoltaic unit can tolerate output currents of two other normalphotovoltaic units, a value of i is 2 or 3.

FIG. 18 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment.

M photovoltaic units are each connected in series to at least one switchinside a DC-DC converter, and then are connected to a direct currentbus.

i photovoltaic units are directly connected in parallel inside the DC-DCconverter. Subsequently, the i photovoltaic units are connected inseries to at least one protection switch and then are connected to thedirect current bus of the DC-DC converter. N is an integer greater thanor equal to 2.

When a short-circuit fault occurs in a single photovoltaic unit, whenthe faulty photovoltaic unit can tolerate an output current of one othernormal photovoltaic unit, a value of i is 2; or when the faultyphotovoltaic unit can tolerate output currents of two other normalphotovoltaic units, a value of i is 2 or 3.

Further, FIG. 19 is a schematic diagram of another photovoltaic powergeneration system according to an embodiment.

A photovoltaic unit and a protective device can be connected in seriesor in parallel, and then be connected to a DC-DC converter. The figureis illustrated by using an example in which every two of i photovoltaicunits are connected in parallel by using a protective device in between.In some embodiments, the protective device may alternatively beconnected in series to a photovoltaic unit.

A protective device Q may be one or a combination of a fuse, anoptimizer, and a disconnection box, or may be another component that canprotect a circuit when a short-circuit fault occurs in the circuit. Thisis not limited in this embodiment.

A value of k in the figure may be determined based on an actual case.This is not limited in this embodiment.

In this case, a release device is further configured to prevent theprotective device from triggering a protection action when controlling aswitching device to be disconnected. In other words, when the currentphotovoltaic power generation system that uses the protective device isreconstructed, the protective device may not need to be removed, and maybe directly connected to the DC-DC converter.

It should be noted that, when a short-circuit fault occurs on a singlephotovoltaic unit, and the faulty photovoltaic unit can tolerate anoutput current of one another normal photovoltaic unit, to avoidtriggering the protection action of the protective device, a value of iis 2. When the faulty photovoltaic unit can tolerate output currents oftwo other normal photovoltaic units, to avoid triggering the protectionaction of the protective device, the value of i is 2 or 3.

FIG. 20 is a schematic diagram of still another photovoltaic powergeneration system according to an embodiment.

The photovoltaic power generation system shown in the figure includes XDC-DC converters 200, and further includes a DC-AC converter 300. TheDC-AC converter may also be referred to as an inverter.

The DC-AC converter and the plurality of DC-DC converters form aninverter 20, which is a string inverter.

Positive output ports of the X DC-DC converters 200 are connected inparallel to a positive input port of the DC-AC converter 300, andnegative output ports of the X DC-DC converters 200 are connected inparallel to a negative input port of the DC-AC converter 300.

An alternating current output by the inverter 20 is aggregated afterflowing through an alternating current combiner box or a switch box, andthen reaches an alternating current power grid after being transformedby a transformer.

FIG. 21 is a schematic diagram of yet another photovoltaic powergeneration system according to an embodiment.

X DC-DC converters 200 included in the photovoltaic power generationsystem shown in the figure form a direct current combiner box 30.Positive output ports of the X DC-DC converters 200 are connected inparallel to form a positive output port of the direct current combinerbox 30. Negative output ports of the X DC-DC converters 200 areconnected in parallel to form a negative output port of the directcurrent combiner box 30.

In some embodiments, the direct current combiner box 30 is an MPPT boostcombiner box, and the positive output end and the negative output end ofthe direct current combiner box 30 are respectively connected to apositive input end and a negative input end of a centralized inverter.

The centralized inverter is configured to convert, into an alternatingcurrent output, a single direct-current input from a direct current sideor a plurality of direct-current inputs that are from the direct currentside and that are connected in parallel. Usually, DC-AC single-stagepower conversion is used. The alternating current output by thecentralized inverter flows through a transformer and then are aggregatedinto an alternating current power grid.

Based on the photovoltaic power generation systems provided in theforegoing embodiments, an embodiment further provides a photovoltaicinverter, which is described below with reference to the accompanyingdrawings.

FIG. 22 is a schematic diagram of a photovoltaic inverter according toan embodiment.

The photovoltaic inverter 20 shown in the figure includes a protectionswitch (not shown in the figure), a DC-AC converter 300, and a pluralityof DC-DC converters 200.

An input end of each DC-DC converter 200 is connected to at least twophotovoltaic units 101, and each photovoltaic unit includes at least onephotovoltaic module.

Each DC-DC converter 200 includes a direct current bus, a DC-DC circuit,and at least one input interface.

Each input interface includes a positive input interface and a negativeinput interface.

The input interface is configured to connect to the photovoltaic units.The positive input interface is connected to a positive direct currentbus inside the photovoltaic inverter 20, and the negative inputinterface is connected to a negative direct current bus inside thephotovoltaic inverter.

Positive output ports of the plurality of DC-DC converters 200 areconnected in parallel to a positive input port of the DC-AC converter300, and negative output ports of the plurality of DC-DC converters 200are connected in parallel to a negative input port of the DC-ACconverter 300.

The protection switch includes a release device and a switching device.The release device is configured to: when a short-circuit fault occurson a line in which the release device is located, control the switchingdevice to be disconnected.

In a possible implementation, the release device is an electromagneticrelease device. When a reverse current on the branch in which therelease device is located is greater than a first current value, therelease device controls the switching device to be disconnected.

In another possible implementation, the release device is anelectromagnetic release device. When an overcurrent occurs on the branchin which the release device is located, the release device controls theswitching device to be disconnected.

In still another possible implementation, the release device is athermal release device. When an overcurrent occurs on the branch inwhich the release device is located, the release device controls theswitching device to be disconnected.

According to the photovoltaic inverter, when a short-circuit faultoccurs on a photovoltaic unit, the release device controls the switchingdevice to be disconnected, so that the interface is disconnected fromthe direct current bus, and the photovoltaic unit connected to theinterface is disconnected from the direct current bus. Therefore, aphotovoltaic unit connected to another interface does not output acurrent to a line in which the photovoltaic unit with the short-circuitfault is located, thereby protecting the photovoltaic unit and the linefrom damage. Based on a protection action triggered by the switchingdevice under control by the release device, no additional controlcircuit is required, and implementation difficulty of the solution isreduced. In addition, because a fuse is no longer used, a Y wire harnessoriginally used for a built-in fuse may be disposed on a photovoltaicunit side instead of being disposed below the photovoltaic inverter ofthe photovoltaic power generation system. Therefore, cable costs arefurther reduced.

Based on the photovoltaic power generation systems provided in theforegoing embodiments, an embodiment further provides a direct currentcombiner box, which is described below with reference to theaccompanying drawings.

FIG. 23 is a schematic diagram of a direct current combiner boxaccording to an embodiment.

The direct current combiner box 30 includes a protection switch (notshown in the figure) and a plurality of DC-DC converters 200.

An input end of each DC-DC converter is connected to at least twophotovoltaic units 101, and each photovoltaic unit includes at least onephotovoltaic module.

Each DC-DC converter includes a direct current bus, a DC-DC circuit, andat least one input interface. The input interface includes a positiveinput interface and a negative input interface. The input interface isconfigured to connect to the photovoltaic units. The positive inputinterface is connected to a positive direct current bus inside thedirect current combiner box 30, and the negative input interface isconnected to a negative direct current bus inside the direct currentcombiner box 30.

Positive output ports of the plurality of DC-DC converters 200 areconnected in parallel to form a positive output port of the directcurrent combiner box 30, and negative output ports of the plurality ofDC-DC converters 200 are connected in parallel to form a negative outputport of the direct current combiner box 30.

The protection switch includes a release device and a switching device.The release device is configured to: when a short-circuit fault occurson a line in which the release device is located, control the switchingdevice to be disconnected.

In a possible implementation, the release device is an electromagneticrelease device. When a reverse current on the branch in which therelease device is located is greater than a first current value, therelease device controls the switching device to be disconnected.

In another possible implementation, the release device is anelectromagnetic release device. When an overcurrent occurs on the branchin which the release device is located, the release device controls theswitching device to be disconnected.

In still another possible implementation, the release device is athermal release device. When an overcurrent occurs on the branch inwhich the release device is located, the release device controls theswitching device to be disconnected.

In some embodiments, at least one of the positive input interface or thenegative input interface of each input interface is connected in seriesto the protection switch inside the direct current combiner box 30.

According to the direct current combiner box, when a short-circuit faultoccurs on a photovoltaic unit, the release device controls the switchingdevice to be disconnected, so that the interface is disconnected fromthe direct current bus, and the photovoltaic unit connected to theinterface is disconnected from the direct current bus. Therefore, aphotovoltaic unit connected to another interface does not output acurrent to a line in which the photovoltaic unit with the short-circuitfault is located, thereby protecting the photovoltaic unit and the linefrom damage. Based on a protection action triggered by the switchingdevice under control by the release device, no additional controlcircuit is required, and implementation difficulty of the solution isreduced. In addition, because a fuse is no longer used, a Y wire harnessoriginally used for a built-in fuse may be disposed on a photovoltaicunit side instead of being disposed below the direct current combinerbox of the photovoltaic power generation system. Therefore, cable costsare further reduced.

“At least one” means one or more, and “a plurality of” means two ormore. The term “and/or” is used to describe an association relationshipbetween associated objects and indicates that three relationships mayexist. For example, “A and/or B” may indicate the following three cases:only A exists, only B exists, and both A and B exist, where A and B maybe singular or plural. The character “/” generally indicates an “or”relationship between the associated objects. “At least one of thefollowing items (pieces)” or a similar expression thereof indicates anycombination of these items, including any combination of singular items(pieces) or plural items (pieces). For example, at least one item(piece) of a, b, or c may represent: a, b, c, “a and b”, “a and c”, “band c”, or “a, b, and c”, where a, b, and c may be singular or plural.

The foregoing embodiments are merely intended to describe the solutions,but are not limiting. Although described in detail with reference to theforegoing embodiments, a person of ordinary skill in the art may stillmake modifications to the solutions described in the foregoingembodiments or make equivalent replacements to some features thereof,without departing from the scope of the embodiments.

What is claimed is:
 1. A photovoltaic power generation system, thephotovoltaic power generation system comprising: a protection switch;and a plurality of DC-DC converters, wherein each DC-DC converter of theplurality of DC-DC converters comprises: a direct current bus, a DC-DCcircuit, and at least one input interfaces configured to connect to aphotovoltaic unit comprising at least one photovoltaic module andconnect to the direct current bus by using at least one protectionswitch, the direct current bus is configured to connect to an input endof the DC-DC circuit, and an output end of the DC-DC circuit is anoutput end of the plurality of DC-DC converters, the at least oneprotection switch comprises a release device and a switching device thatare connected in series, the release device is configured to release aholding mechanism when a protection action is triggered due to ashort-circuit fault that occurs on a line in which the release device islocated, and to disconnect the switching device so as to protect thephotovoltaic unit, the line, or the photovoltaic power generationsystem.
 2. The photovoltaic power generation system according to claim1, wherein the release device is an electromagnetic release device, theshort-circuit fault comprises either a reverse current, on the line inwhich the release device is located, that is greater than a firstcurrent value or an overcurrent that occurs on the line in which therelease device is located, and the release device is configured tocontrol the switching device to be disconnected.
 3. The photovoltaicpower generation system according to claim 1, wherein the release deviceis an electromagnetic release device, and when an overcurrent occurs onthe line in which the release device is located, the release device isconfigured to control the switching device to be disconnected.
 4. Thephotovoltaic power generation system according to claim 1, wherein therelease device is a thermal release device, and when an overcurrentoccurs on the line in which the release device is located, the releasedevice is configured to control the switching device to be disconnected.5. The photovoltaic power generation system according to claim 1,wherein there are a plurality of input interfaces and a plurality ofphotovoltaic units, and each input interface is connected to arespective photovoltaic unit.
 6. The photovoltaic power generationsystem according to claim 1, wherein a maximum of two photovoltaic unitsare connected in parallel, and then connected to the at least one inputinterface.
 7. The photovoltaic power generation system according toclaim 1, wherein a maximum of three photovoltaic units are connected inparallel, and then connected to the at least one input interface.
 8. Thephotovoltaic power generation system according to claim 1, wherein thephotovoltaic power generation system further comprises a DC-ACconverter, the DC-AC converter and the plurality of DC-DC convertersform an inverter, positive output ports of the plurality of DC-DCconverters are connected in parallel to a positive input port of theDC-AC converter, and negative output ports of the plurality of DC-DCconverters are connected in parallel to a negative input port of theDC-AC converter.
 9. The photovoltaic power generation system accordingto claim 1, wherein the plurality of DC-DC converters forms a directcurrent combiner box; and positive output ports of the plurality ofDC-DC converters are connected in parallel to form a positive outputport of the direct current combiner box; and negative output ports ofthe plurality of DC-DC converters are connected in parallel to form anegative output port of the direct current combiner box.
 10. Thephotovoltaic power generation system according to claim 1, furthercomprising: a protective device connected in series or in parallel tothe photovoltaic unit, wherein the release device is further configuredto when controlling the switching device to be disconnected, prevent theprotective device from triggering a protection action.
 11. Thephotovoltaic power generation system according to claim 10, wherein theprotective device comprises at least one of the following: a fuse, anoptimizer, and a disconnection box.
 12. A photovoltaic inverter,configured to connect to a photovoltaic unit, wherein the photovoltaicunit comprises at least one photovoltaic module, and the photovoltaicinverter comprises: a protection switch; a DC-AC converter; and aplurality of DC-DC converters, wherein each DC-DC converter of theplurality of DC-DC converters comprises: a direct current bus, a DC-DCcircuit, and at least one input interface configured to connect to thephotovoltaic unit, the photovoltaic unit and connect to the directcurrent bus by using the protection switch, the direct current bus isconfigured to connect to an input end of the DC-DC circuit, and anoutput end of the DC-DC circuit is an output end of the DC-DC converter,positive ports of the output ends of the plurality of DC-DC convertersare connected in parallel to a positive input port of the DC-ACconverter, negative ports of the output ends of the plurality of DC-DCconverters are connected in parallel to a negative input port of theDC-AC converter, the protection switch comprises a release device and aswitching device that are connected in series, and the release device isconfigured to release a holding mechanism when a protection action istriggered due to a short-circuit fault that occurs on a line in whichthe release device is located, to disconnect the switching device so asto protect the photovoltaic unit, the line, or a power generationsystem.
 13. The photovoltaic inverter according to claim 12, wherein therelease device is an electromagnetic release device, and when a reversecurrent on the line in which the release device is located is greaterthan a first current value or an overcurrent occurs on the line in whichthe release device is located, the release device is configured tocontrol the switching device to be disconnected.
 14. The photovoltaicinverter according to claim 12, wherein the release device is anelectromagnetic release device, and when an overcurrent occurs on theline in which the release device is located, the release device isconfigured to control the switching device to be disconnected.
 15. Thephotovoltaic inverter according to claim 14, wherein the release deviceis a thermal release device, and when an overcurrent occurs on the linein which the release device is located, the release device is configuredto control the switching device to be disconnected.
 16. A protectioncontrol method for a power converter which configured to connect to aphotovoltaic unit with at least one photovoltaic module, and the powerconverter comprises a protection switch and a plurality of DC-DCconverters, each DC-DC converter of the plurality of DC-DC converterscomprises a direct current bus, a DC-DC circuit, and at least one inputinterface configured to connect to the photovoltaic unit and connect tothe direct current bus by using the protection switch, the directcurrent bus is configured to connect to an input end of the DC-DCcircuit, and an output end of the DC-DC circuit is an output end of theDC-DC converter, positive ports of the output ends of the plurality ofDC-DC converters are connected in parallel to a positive output port ofthe power converter, negative ports of the output ends of the pluralityof DC-DC converters are connected in parallel to a negative output portof the power converter, the protection switch comprises a release deviceand a switching device that are connected in series; and the methodcomprises: releasing a holding mechanism when a protection action istriggered due to a short-circuit fault that occurs on a line in whichthe release device is located, to disconnect the switching device so asto protect the photovoltaic unit, the line. or the power converter. 17.The method according to claim 16, wherein the release device is anelectromagnetic release device, and, when a reverse current on the linein which the release device is located is greater than a first currentvalue, the release device disconnects the switching device.
 18. Themethod according to claim 16, wherein the release device is anelectromagnetic release device, and, when an overcurrent occurs on theline in which the release device is located, the release devicedisconnects the switching device.
 19. The method according to claim 16,wherein the release device is a thermal release device, and, when anovercurrent occurs on the line in which the release device is located,the release device disconnects the switching device.
 20. The methodaccording to claim 16, wherein the short-circuit fault occurs when areverse current on the line in which the release device is located isgreater than a first current value.